This invention relates to products for absorbing aqueous fluids and, in particular, products for absorbing body fluids. The invention is specifically directed toward compositions for producing crosslinked polyelectrolytes which compositions are usable in commercial products for absorbing aqueous liquids and are particularly useful in producing absorbent composite materials which are coated or impregnated with such compositions and subsequently treated so as to produce a composite material incorporating polyelectrolyte which is crosslinked in situ.
Highly absorbent crosslinked polyelectrolytes and methods for preparing the same are already known. U.S. Pat. Nos. 3,669,103 and 3,670,731 teach the use of these materials in diapers and dressings. U.S. Pat. Nos. 2,988,539; 3,393,168; 3,514,419 and 3,557,067 teach methods of making such absorbents and, in particular, are related to making water swellable crosslinked carboxylic copolymers that are either crosslinked during copolymerization or crosslinked after polymerization and then neutralized to result in pendant ionic moieties capable of imparting water retention properties to a finished material.
Further, the process of incorporating such polyelectrolytes or their monomeric precursors into a substrate such as a fibrous web or a cellular, i.e. foam, material and then crosslinking in situ, is also known. Published European Patent Office Application 81302086.4 discloses impregnating a fibrous web with a solution of monomers, with or without crosslinking agents and then irradiating such impregnated web to polymerizing and/or crosslink in situ. U.S. Pat. Nos. 4,076,673; 3,980,663; 4,071,650 all exemplify teachings of preparing solutions for coating substrates which subsequently may be treated to produce in situ crosslinked polyelectrolytes.
Unfortunately, these foregoing suggestions suffer from several drawbacks when it is contemplated to translate such suggestions into commercial processes. Suggestions directed toward irradiation techniques, when scaled to commercial proportions, require large capital investment for irradiating equipment such as, for example, an electron beam device, and, concommitantly, consume large quantities of energy for carrying out such processes. Additionally, in order to obtain high conversion of monomer to polymer, such irradiation technique requires still more energy input and also tend to over crosslink the polymer resulting in a decrease in absorption capacity.
Those suggestions relating to preparing compositions comprising polyelectrolytes and crosslinking agents and then applying such composition to a substrate are also fraught with problems on a commercial level A major drawback to such systems is the limited shelf life of the impregnating or coating concentration Almost immediately upon mixing the polyelectrolyte with the crosslinker, a reaction begins that leads to gel formation. Once gelled the composition can no longer be used. Accordingly, a commercial process must, to utilize such suggestions, apply the components i.e., the polyelectrolyte and the crosslinker, in separate steps and hence suffer the concommitant inconvenience as well as the likelihood of locally applied non-uniform proportions of these components. The problem is compounded when such commercial processes are designed to impregnate a substrate with excess composition and then recirculate to reuse the excess. Such a system will be encountered when attempts are made to impregnate a fibrous or cellular web with vacuum deposition techniques such as are common in the art of producing bonded non-woven fabrics. In such a process, of necessity, the active components are intermixed and gelling follows.
Accordingly, there is a need for providing a commercial process for in situ crosslinking of polyelectrolytes to make absorbents, and especially superabsorbents.
The most pertinent prior art reference, Gross, U.S. Pat. No. 4,079,029, discloses one possible way of doing this, involving: a solution useful to form water swellable articles of a carboxylic synthetic polyelectrolyte upon curing which comprises
1. a solvent selected from the group consisting of water, lower alcohols and mixtures thereof,
2. about 5 to about 60%, by weight based on (1), of a crosslinkable carboxylic copolymer which contains in the copolymer
(A) about 25 to about 98% by weight based on the total weight of the copolymer of an alkali metal salt of an olefinically unsaturated monocarboxylic acid;
(B) about 2 to about 50% by weight of an olefinically unsaturated monocarboxylic acid;
(C) about 25 to about 60% by weight of an alkyl ester of an olefinically unsaturated monocarboxylic acid and
(D) about 0.3 to about 5.0% by weight of crosslinking units of an N-substituted acrylamide or methacrylamide having the formula H.sub.2 C.dbd.CR--C(O)--NHCH.sub.2 --O--R.sup.1 wherein R is selected from hydrogen or methyl and R.sup.1 is hydrogen or an alkyl group of 1-8 carbons. The Gross patent specifically listed the following as examples of the N-hydroxymethyl or N-alkoxymethylene acrylamides or methacrylamides usable there:
N-methoxymethyl acrylamide, PA0 N-propoxymethyl acrylamide, PA0 N-isopropoxymethyl acrylamide, PA0 N-ethoxymethyl acrylamide, PA0 N-methylol acrylamide, PA0 N-butoxymethyl acrylamide, and PA0 N-tertiary butoxy methyl acrylamide, PA0 N-isobutoxymethyl acrylamide, PA0 N-octyloxymethyl acrylamide, PA0 N-methoxymethyl methacrylamide, PA0 N-propoxymethyl methacrylamide, PA0 N-isopropoxymethyl methacrylamide, PA0 N-ethoxymethyl methacrylamide, PA0 N-methylol methacrylamide, PA0 N-butoxymethyl methacrylamide, PA0 N-tertiary butoxy methyl methacrylamide, PA0 N-isobutoxymethyl methacrylamide, and PA0 N-octyloxymethyl methacrylamide.
In said Gross U.S. Pat. No. 4,079,029, the method of making water swellable films involves starting with the above composition of the polyelectrolytes having a pH range from 7 to 9 which subsequently is acidified by adding organic or inorganic acids to a pH range 3 to 6 and spread on a flat plate or roller of metal, plastic, or other impervious substrate and heated to a temperature greater than 30.degree. C. to crosslink the polyelectrolyte and drive off the excess water and/or alcohol. The film is then peeled off the plate or roller by a scraper to recover the intact film for subsequent storage or use.
The present invention takes an opposite approach from said Gross patent, by starting with polyelectrolytes having an acid pH range and then increasing the pH (in contrast to Gross which starts with polyelectrolytes having a basic pH range and then decreasing the pH). The present invention uses a broader (but overlapping) reactive self crosslinking monomer than does Gross. It was defined in the patent application Ser. No. 749,907 of this application as one having the formula: ##STR1## wherein
R is H or CH.sub.3, n is O or 1-5, and X is a group capable of reacting through self condensation or with carboxylic acid by condensation or ring opening, preferably wherein X is glycidyl, N-methylol, N-methylol alkyl ether having alkyl groups of 1-4 carbons, blocked isocyanate, N-methylol urethane, or aziridine.
Of the above possible definitions of X, said Gross patent
discloses
X.dbd.N-methylol and X.dbd.N-methyol alkyl ether having alkyl groups of 1-4 carbons, and thus overlapped to that extent only.
The Gross patent calls for a polyelectrolyte involving a copolymer having 4 different monomeric units, designated above as 2A., B., C. and D. Applicant's comparable polyelectrolyte at first appears different by involving a copolymer having only 3 different monomers, by lacking monomeric unit 2B which Gross defines as "about 2 to about 50% of an olefinically unsaturated monocarboxylic acid". However, under certain pH conditions (e.g. at pH5) Applicant's monomer comparable to Gross' monomeric unit 2A..."an alkali metal salt of an olefinically unsaturated monocarboxylic acid" exists in equilibrium with the free acid, so that some of Applicant's polyelectrolytes overlap with those of Gross under certain limited conditions.
The presence of free carboxylic acid is a function of the pH of the polymer solution. At pH=5 about 50% of a polyacrylic acid polymer is in the acidic form. However, by increasing the pH to 9 all of the acid groups are converted to the carboxylate salt. The crosslinking reaction, described in Gross U.S. Pat. No. 4,079,029 will not take place unless the pH is reduced to below pH=6. This is described in column 3 lines 27-28, and is reflected in the Gross patent claim 2B as a free carboxylic acid, 2% to 50% by weight.
In the present invention, in contrast to Gross, the best mode of making the superabsorbent involves increasing the pH to 9 with ammonium hydroxide solution, followed by drying and crosslinking. If this is done with sodium hydroxide, no crosslinking takes place (as demonstrated in Example II below) and no superabsorbent is formed. Additionally, a significantly more absorbent polymer is made from the pH=9 solution as compared to the pH=5 (see examples I, II below). Thus, a key difference between this invention of this application and Gross U.S. Pat. No. 4,079,029, is, that here the solution, from which an absorbent polymer is prepared, should have a pH greater than 7, preferably 9, and that at least 50% (by mole) of the neutralizing base must be ammonium hydroxide. The alkaline pH also increases the shelf life of the solution in the present invention.